Working in C# like its Go

Go.cs

public record User(int Id, string Username);

// might have some state, doesnt necessarily need any though
public record UserController();

public static class UserControllerExt {
  public static void DoComplexActionAgainstUser(
    this UserController uc,
    IUserConn conn,
    int id) {
    // do things against conn
  }
}

public interface IUserConn {
  User Get(int id);
  User Put(User user);
  User Patch(User user);
  User Delete(User user);
}

public record UserConn(DbConnection DbConnection) : IUserConn {
  // implementation details, maybe dapper crud ;)
}

IUserConn GetMockUserConn() {
  // get in-memory or on-disk mock db or something
}

void TestAction() {
  var conn = GetMockUserConn();
  var user = conn.Put(new User("name"));

  var controller = new UserController();
  controller.DoComplexActionAgainstUser(conn, user.Id);

  var updatedUser = conn.Get(user.Id);
  // assert some state
}